Various types of downhole tools employ slips which expand radially outward to engage the interior of a downhole tubular, thereby securing the tool in the well. Various types of slip mechanisms have been used for this purpose, with slips conventionally having outer teeth which bite into the inner surface of the downhole tubular to secure the tool in the well.
One of the significant problems with downhole tools having slip mechanisms is that the slip mechanism may be inadvertently actuated before actuation is intended, in which case the slip or slips may radially expand and engage the tubular. It may then be difficult or practically impossible in some instances to break the connection between the downhole tool and the tubular, and accordingly the tool cannot be easily retrieved to the surface, repaired, and then again reinserted into the well. In other instances, the slip may be expanded when the tool is at a desired downhole position, but the running tool/set tool connection cannot be reliably released, so that the retrievable portion of the tool cannot be returned to the surface.
U.S. Pat. Nos. 3,920,057 and 4,281,711 disclose a liner hanger assembly for hanging a liner in a well. A liner hanger assembly is also disclosed in U.S. Pat. No. 6,739,398. The '398 patent discloses a tool that relies upon shear pins to keep the slips from prematurely releasing while the tool is run in the hole. While pushing on the running tool may not release the slip accidentally, if debris were to build up around the tieback or other components of the liner hanger and the operator then pushed or pulled on the tool, the shear pins may shear and thereby release the slip. Tools that allow axial forces to be transmitted to the tool through the running string to shear pins and release the slip may thus inadvertently be actuated when running the tool to the desired depth in the well.
U.S. Pat. Nos. 4,712,614, 4,603,743, 4,311,194 and 4,287,949 also rely on shear pins to keep from releasing the slips. U.S. Pat. No. 5,318,131 is another example of the downhole tool using shear pins to prevent release of slips. Downhole tools with shear pins to keep the slip in the retracted position while running the tool in the hole generally have exposed hydraulic actuation ports. If debris builds up around the tool while tripping into the hole, the dragging action could shear the pins, in which case the slip will set and thereby prevent the tool from being positioned at its desired location in the well. Increased hydraulic pressure may also cause the pins to prematurely shear.
The disadvantages of the prior art are overcome by the present invention, and a liner hanger with an improved slip releasing mechanism is hereinafter disclosed.